全文获取类型
收费全文 | 836篇 |
免费 | 50篇 |
国内免费 | 3篇 |
专业分类
化学 | 612篇 |
晶体学 | 4篇 |
力学 | 4篇 |
数学 | 152篇 |
物理学 | 117篇 |
出版年
2023年 | 8篇 |
2022年 | 8篇 |
2021年 | 19篇 |
2020年 | 17篇 |
2019年 | 27篇 |
2018年 | 12篇 |
2017年 | 9篇 |
2016年 | 34篇 |
2015年 | 20篇 |
2014年 | 30篇 |
2013年 | 37篇 |
2012年 | 36篇 |
2011年 | 41篇 |
2010年 | 27篇 |
2009年 | 31篇 |
2008年 | 56篇 |
2007年 | 37篇 |
2006年 | 45篇 |
2005年 | 52篇 |
2004年 | 30篇 |
2003年 | 28篇 |
2002年 | 24篇 |
2001年 | 11篇 |
2000年 | 9篇 |
1999年 | 7篇 |
1998年 | 9篇 |
1997年 | 5篇 |
1996年 | 11篇 |
1995年 | 5篇 |
1994年 | 10篇 |
1993年 | 8篇 |
1991年 | 11篇 |
1990年 | 7篇 |
1989年 | 5篇 |
1988年 | 10篇 |
1986年 | 10篇 |
1985年 | 11篇 |
1984年 | 13篇 |
1983年 | 11篇 |
1982年 | 7篇 |
1981年 | 9篇 |
1980年 | 9篇 |
1979年 | 13篇 |
1978年 | 14篇 |
1977年 | 6篇 |
1976年 | 9篇 |
1975年 | 6篇 |
1974年 | 3篇 |
1973年 | 5篇 |
1971年 | 5篇 |
排序方式: 共有889条查询结果,搜索用时 546 毫秒
71.
The knapsack problem, maximize Σmi = 1cixi when Σmi = 1aixi?b for integers xi?0, can be solved by the classical step-off algorithm. The algorithm develops a series of feasible solutions with ever-increasing objective values. We make a change in the problem so that the step-off algorithm produces a series of solutions of not necessarily increasing objective values. A point is reached when no better solutions can be found and the calculation is stopped. 相似文献
72.
Bidentate intermolecular Si...N interactions were utilized to form new hypervalent complexes of trifluoro-phenylethynyl-silane with 2,2'-bipyridine and with 1,10-phenanthroline. X-ray structures obtained for these complexes display a somewhat distorted octahedral geometry about the silicon atom. Binding constants ranging from 170 to 1600 M(-1) at 25 degrees C in CDCl3 were measured for the formation of these complexes, suggesting that such hypervalent complexes of silicon could be used as new motifs in supramolecular chemistry. 相似文献
73.
Fluorescent optical imaging of tumors deep within tissue depends on specific binding of antibodies to the tumors' surface markers. These fluorescent antibodies propagating in the vicinity of the tumor can be attached to and (or) diffused away from it. We illustrate application of a new tool, based on the random-walk theory in turbid media, for extracting the pharmacokinetics of these fluorescent antibodies by data deconvolution, excluding the effect of upper turbid tissue layers. 相似文献
74.
Bella Dolgin Valery Bulatov Israel Schechter 《Analytical and bioanalytical chemistry》2009,395(7):2151-2159
A nondestructive method for quantitative parchment characterization and sensitive indication of its deterioration stage was
developed. Synchronous fluorescence (SF) measurements were applied for the first time to parchment samples. The method provides
detailed spectral features, which are useful for parchment characterization. The discrimination of parchment samples into
groups (modern, historical, and artificially aged) was successfully performed. The SF spectra could be resolved into specific
fluorophores, which were related to the parchment condition. The spectral data indicate a continuous change in the collagen-to-gelatin
ratio during the aging process. Depth-resolved synchronous fluorescence spectra were also measured. The data indicate that
parchments possess a layered structure, and the dominant fluorophore in the upper layer is different from those in the lower
layers. Layer-resolved profiling allows for quantifying the contribution of each fluorophore in each given layer. This way,
significant differences between modern, artificially aged, and historical samples can be observed. 相似文献
75.
76.
77.
78.
Federico Chiariotti Israel Leyva-Mayorga edomir Stefanovi Anders E. Kalr Petar Popovski 《Entropy (Basel, Switzerland)》2021,23(6)
Wireless mobile networks from the fifth generation (5G) and beyond serve as platforms for flexible support of heterogeneous traffic types with diverse performance requirements. In particular, the broadband services aim for the traditional rate optimization, while the time-sensitive services aim for the optimization of latency and reliability, and some novel metrics such as Age of Information (AoI). In such settings, the key question is the one of spectrum slicing: how these services share the same chunk of available spectrum while meeting the heterogeneous requirements. In this work we investigated the two canonical frameworks for spectrum sharing, Orthogonal Multiple Access (OMA) and Non-Orthogonal Multiple Access (NOMA), in a simple, but insightful setup with a single time-slotted shared frequency channel, involving one broadband user, aiming to maximize throughput and using packet-level coding to protect its transmissions from noise and interference, and several intermittent users, aiming to either to improve their latency-reliability performance or to minimize their AoI. We analytically assessed the performances of Time Division Multiple Access (TDMA) and ALOHA-based schemes in both OMA and NOMA frameworks by deriving their Pareto regions and the corresponding optimal values of their parameters. Our results show that NOMA can outperform traditional OMA in latency-reliability oriented systems in most conditions, but OMA performs slightly better in age-oriented systems. 相似文献
79.
Wang Y Fernández I Duvall M Wu JI Li Q Frenking G Schleyer PR 《The Journal of organic chemistry》2010,75(23):8252-8257
Quantitative evaluations of the aromaticity (antiaromaticity) of neutral exocyclic substituted cyclopropenes (HC)(2)C=X (X = BH to InH (group 13), CH(2) to SnH(2) (group 14), NH to SbH (group 15), O to Te (group 16)) by their computed extra cyclic resonance energies (ECRE, via the block-localized wave function method) and by their aromatic stabilization energies (ASEs, via energy decomposition analyses) correlate satisfactorily (R(2) = 0.974). Electronegative X-based substituents increase the aromaticity of the cyclopropene rings, whereas electropositive substituents have the opposite effect. For example, (HC)(2)C=O is the most aromatic (ECRE = 10.3 kcal/mol), and (HC)(2)C=InH is the most antiaromatic (ECRE = -15.0 kcal/mol). The most refined dissected nucleus-independent chemical shift magnetic aromaticity index, NICS(0)(πzz), also agrees with both energetic indexes (R(2) = 0.968, for ECRE; R(2) = 0.974, for ASE), as do anisotropy of the induced current density plots. 相似文献
80.
Israel Quiros Ricardo García-Salcedo Tonatiuh Matos Claudia Moreno 《Physics letters. [Part B]》2009,670(4-5):259-265
We apply the dynamical systems tools to study the linear dynamics of a self-interacting scalar field trapped on a DGP brane. The simplest kinds of self-interaction potentials are investigated: (a) constant potential, and (b) exponential potential. It is shown that the dynamics of DGP models can be very rich and complex. One of the most interesting results of this study shows that dynamical screening of the scalar field self-interaction potential, occurring within the Minkowski cosmological phase of the DGP model and that mimics 4D phantom behaviour, is an attractor solution for a constant self-interaction potential but not for the exponential one. In the latter case gravitational screening is not even a critical point of the corresponding autonomous system of ordinary differential equations. 相似文献